1. Field of the Invention
The present invention relates to synchronous motor control and excitation systems, and more particularly to a control system for a brushless exciter which includes a post-synchronizing firing circuit and means for inhibiting the operation of the post-synchronizing firing circuit during subsynchronous operation.
2. Description of the Prior Art
Generally, the excitation system for a synchronous motor includes a field discharge circuit for discharging induced field current during the start-up period and a DC excitation circuit for energizing the motor field winding at synchronous speeds as well as during the predetermined terminal interval of the start-up period to cause the motor to pull into synchronism. The pre-synchronizing application of DC excitation to the field winding is ordinarily necessary to develop the pull-in torque required to synchronize the motor. Just before or after synchronization, a field discharge circuit must be opened or removed from operation so as to avoid current drain from the DC excitation circuit. For greater detail on the theory of synchronization, reference is made to U.S. Pat. No. 3,405,338, "Brushless Synchronous Motor Control System And Circuitry Therefor", issued to F. V. Frola.
It is desirable that the DC excitation be applied at a predetermined point in time or in the slip voltage waveform, or at least a predetermined slip voltage frequency and within a certain phase range of the slip voltage cycle at that frequency. In some applications where the motor is accelerated with zero or light load, the motor can be synchronized without the application of DC excitation during the slip period. Post-synchronizing circuitry then can be employed for applying the DC excitation after synchronization is achieved. Circuitry for this purpose, which includes a post-synchronizing timing capacitor, is disclosed in U.S. Pat. No. 3,381,195, "Excitation Control System For Synchronous Motors", issued to A. H. Hoffmann. Normally, however, DC excitation must be applied during the slip period for synchronism to be achieved.
In the above-mentioned Frola Patent, circuitry is employed for sharply firing a semiconductor exciter switch in the excitation circuit at a predetermined slip voltage frequency and just as the slip voltage is reversing from a positive to a negative polarity. Inherent characteristics of the firing circuit produce the accurately timed firing of the exciter switch. Since the exciter switch is fired just as the slip voltage passes through zero to a negative value, there is some degree of probability that the semiconductor field discharge switch in the field discharge circuit may continue to conduct after the exciter switch is turned on. Some of the current in the excitation circuit is then drained through the field discharge switch and shunted from the motor field winding. Since the magnitude of the drainage current from the excitation circuit can be substantial, it is normally necessary that separate field resistor removal circuitry be employed to open the field discharge switch within a short interval of time.
The need for special field resistor removal circuitry is eliminated through the employment of a circuit which fires the exciter switch after the field discharge switch is open, as described in U.S. Pat. No. 3,414,788, "Excitation Control System For Synchronous Motors", issued to A. H. Hoffmann et al. Synchronism is achieved by firing a gating circuit which provides a sharp signal or pulse in response to the frequency of induced or slip voltage across the field winding, and the exciter switch is closed at substantially predetermined slip voltage frequency and within the best phase range of the slip voltage cycle at that frequency. The particular phase point at which the exciter switch is fired assures prior reversal of polarity across the field discharge switch so that discharge switch is open when DC exciting current starts to flow through the exciter switch to energize the field winding means. Any need for special field resistor removal circuitry is thus eliminated.
Under usual operating conditions of rated stator voltage and low inertia the previous practices have proved to be reliable and satisfactory. However, under conditions of low stator starting voltage, as encountered in shop testing procedures or in installation under weak system conditions, the previous circuitry has allowed the post-synchronizing timing capacitor to charge while the machine was accelerating, sometimes causing application of the field before the machine reaches synchronous speed. This condition causes the starting resistor to remain in parallel across the field of the machine causing overloading of the machine's exciter and semiconductor components with subsequent loss of operation of the motor.